This invention relates to the recovery of boric acid from aqueous solutions. More specifically, this invention relates to an improvement in the method for recovering boric acid from an anion exchange resin.
Boron, or more specifically, boron.sup.10 has a high-capture cross section for thermal neutrons and is used as a chemical neutron absorber for "hot to cold" shutdown of some types of nuclear reactors. At present, boric acid which normally contains about 20% .sup.10 B is the most useful chemical for this purpose, since it has sufficient solubility, is relatively inexpensive and is readily removed. For certain types of reactors, boric acids containing a much higher percentage of .sup.10 B have been prepared, which are very expensive.
In order for the reactor to go critical and restart, the boric acid must be removed from the main coolant. This can be accomplished by passing the coolant water through an appropriate anion exchange resin especially purified for nuclear applications.
Replacement of the anion exchange resin is expensive and elution of the boric acid from the resin is inefficient or results in contamination of the boric acid which requires additional purification before it can be reused as a nuclear reactor poison.
For example, an aqueous ammonia solution has been suggested as an ideal eluent for the boric acid, since the ammoniacal eluate can be boiled to drive off the ammonia. However, it was found that a 1 N ammonia hydroxide solution was very ineffective in stripping the boric acid and had a high pH which made it undesirable for use with aluminum vessels.
Boric acid held by a strong-base ion exchange resin can be displaced by anions such as chloride, nitrate or sulfate. However, when any of these are present, concentrating the eluate without loss of boric acid is difficult. If, for example, hydrochloric acid is used to strip the boron from the resin, concentrating the solution by distillation is not feasible, as boric acid will volatilize. If sodium or ammonium chloride is used, then a large salt residue will remain in the concentrator with the boric acid and cause an additional difficulty in recovery of the boric acid.